This invention relates to an air-fuel ratio control apparatus operative to automatically effect control of the air-fuel ratio of fuel-air mixtures supplied from a carburetor to a gasoline engine in such a manner that the ratio can be automatically brought to a predetermined level.
In order to avoid the problem of air polution by exhaust emissions of internal combustion engines, it is desired that the air-fuel ratios of fuel-air mixtures supplied to the internal combustion engines be kept constant. Particularly when a motor vehicle uses an exhaust emission control system in which a so-called ternary catalyst is employed for oxidating carbon monoxide and hydrocarbons and at the same time reducing oxides of nitrogen by means of the single catalyst, the air-fuel ratio of the fuel-air mixtures supplied to the engine is preferably maintained in a narrow range or in a range of values which is .+-.0.2 of the theoretical air-fuel ratio, for example. This is because, if the air-fuel ratios are confined to the indicated range, the carbon monoxide, hydrocarbons and oxides of nitrogen in the exhaust emissions will all be controlled at a percentage of over 90% by means of a ternary catalyst, and thus the ternary catalyst can achieve excellent results in exhaust emission control.
However, in carburetors currently used in motor vehicles, a great difficulty is encountered in restricting the changes in the air-fuel ratio to a narrow range of values slightly greater and smaller than the theoretical air-fuel ratio, while the engine is operating at normal engine speeds.
To cope with this situation, proposals have been made to use air-fuel ratio control apparatus of the feedback system which detect the concentration of exhaust gases from the engine and effect control of the air-fuel ratio of fuel-air mixtures supplied to the engine in conformity with a detection signal. Such apparatus are disclosed, for example, in U.S. Pat. Nos. 3,942,493, 3,960,118, Japanese Laid-Open Patent Publication Nos. 54131/76 and 20654/76.
In one type of air-fuel ratio control apparatus of the feedback system known in the art, electronic fuel injecting means is used which detects the concentration of oxygen in exhaust gases and injects fuel into the air introduced into the engine, in such a manner that the volume of the injected fuel is determined by the detected concentration of oxygen. This type of apparatus can achieve good effects in controlling the volume of the fuel injected into the air, but has the disadvantage of greatly increased in production cost as compared with conventional carburetors of the fixed Venturi type.
Another type of air-fuel ratio control apparatus of the feedback system known in the art, which is used with a carburetor of the fixed Venturi type, operates in a manner to control the volume of fuel supplied to the carburetor of the fixed Venturi type in accordance with the concentration of oxygen in the exhaust gases. This type of apparatus is disclosed in Japanese Laid-Open Patent Publication No. 54131/76. In this patent publication, the fixed Venturi type carburetor comprises a main fuel supply system and a low speed fuel supply system, and the two fuel supply systems each have an air bleed and an on-off electromagnetic valve for controlling the volume of air flowing through the respective air bleed. The two electromagnetic valves are controlled in accordance with the centration of oxygen in the exhaust gases individually and independently of each other. Thus this apparatus has the disadvantage of a control circuit for the electromagnetic valves becoming complex in construction.
On the other hand, from the point of view of operation of the engine, it is not desirable to keep constant the air-fuel ratio at all engine operating conditions. More specifically, when the engine is started while it is still in cold condition, vaporization of the fuel does not take place vigorously and the fuel-air mixtures supplied in the form of a gas to the combustion engine is very lean. Therefore, it is necessary to supply to the engine a richer fuel-air mixture when the engine is started in cold condition than when it is started after being warmed up in the usual manner, to maintain the mixture in a combustible range. When the motor vehicle runs in power operating and high speed operating conditions of the engine, it is also desired that fuel-air mixtures of an air-fuel ratio lower than the theoretical air-fuel ratio be supplied to the engine. None of the air-fuel ratio control apparatus of the prior art can satisfy the aforementioned demands of the engine.